The invention relates to a method and a system for verifying a speaker by a computer.
In a speaker verification, a speaker is to be identified or authenticated to a system, for example during an access check, by using his voice. In the verification, it is of particular importance to also ensure that the correct user is speaking and the characteristics of the speaker thus authenticate him to the system.
A device suitable for this purpose is known from DE 199 07 854 A1. In this document, it is also proposed that the speaker is only verified if the quality of the code word input is above a predetermined threshold of acceptance.
Problems are presented by the fact that for an unauthorized user (attacker) to outsmart the system (e.g. the access check), it may be sufficient to record the password input of the authorized user and to gain unauthorized access by replaying the recording. This attack is particularly simple when each speaker who authenticates himself to the system has his own password which is compared with patterns of the same password of the same speaker in the form of speech and access to the system is granted if the match is sufficiently good.
It is thus disadvantageous that a relatively simple attack of the unauthorized person can lead to success and thus contributes to a quite low acceptance of the verification of the speaker by his utterance.
In Furui S. et al. “Phoneme-Level Voice Individuality Used in Speaker Recognition”, Yokohama, Japan, 1994, Yokohama: ASJ, pages 1463-1466, a method for verifying a speaker by a computer is specified in which phonemes are used, that is to say the smallest semantically distinguishable sound units. As a result, the possible vocabulary which a speaker is allowed to speak is not predetermined. On the other hand, using phonemes according to the method disclosed makes it mandatory to use relatively computationally complicated methods such as, for example, HMMs in order to model the speakers.